Irradiation of most substances results in the emission of secondary or fluorescent x-rays having wavelengths or energies that are determined by the kinds of chemical elements that are present in the substance. Each element fluoreses x-rays having a different energy or combination of energies which are known as the characteristic x-rays of the element. Consequently it is possible to detect the presence and amounts of particular elements in a substance by irradiating the substance and then analyzing the energies of the resulting fluorescent x-rays.
In some cases it is desirable not only to detect the presence of one or more elements but also to determine the spatial distribution of such elements within an object or region.
In the quality control of manufactured products, such as printed circuits for example, it may be desirable to check the uniformity of metallic plating on a base material. Detection of variations in the concentration of an element within an object may also be highly useful in the analysis of ore samples. Considering still another example, certain elements tend to concentrate in certain specific regions of the human body. Iodine, for example, concentrates in the thyroid gland and it can be useful for medical diagnostic purposes to obtain an image showing variations in the distribution of the iodine within the gland. Information about the distribution of chemical elements in objects, samples and specimens of various kinds is also useful for a variety of other purposes.
To detect the distribution of one or more elements within an object by x-ray fluorescence analysis, the primary radiation is directed to only a localized small area of the object at any given time. The small irradiation area is traveled along a scan path on the object and x-ray fluorescence emitted from successive points along the scan path is detected and analyzed to identify x-rays having energies characteristic of the element or elements of interest. A uniform count of characteristic x-rays during the scanning operation indicates a uniform distribution of the corresponding element. Variations in the rate of detection of the characteristic x-rays during the scan indicate corresponding variations in the concentration of the element within the scanned region.
A visible display of the distribution of the element may be produced by synchronizing the raster control of a cathode ray tube or the like with the scanning motion at the object while modulating the intensity control of the tube in accordance with the characteristic x-ray detection rate. Where the display is to depict the distribution of more than one element, the characteristic x-ray count rate signal for each element may be used to modulate a different one of the color intensity controls of a cathode ray tube which presents multi-color displays. Alternately, the distribution data may be processed by a graphical printer or may be digitized and printed out in alphanumeric form.
Scanning x-ray spectrometers have been subject to serious problems and limitations that derive from the systems heretofore used to irradiate only a small area of the object which is being analyzed and to travel the area of irradiation along a scan path on the object.
In the first general type scanning x-ray spectrometer the primary radiation which excites x-ray fluorescence is an electron beam. The beam is swept along the scan path, typically through a raster pattern of parallel scan lines, by magnetic or electrostatic deflection means. As a practical matter, a scanning electron microscope structure is usually adapted to perform the scanning operation.
Electrons do not penetrate very far into the object which is being examined. Consequently only the distribution of an element near the surface can be detected. A further limitation is present in that the object must be an electrical conductor. If it is not a conductor, electrical charge builds up on the surface of the object and causes distortions in the operation of the system and in the output data. In some cases, non-conductors can be analyzed by applying a thin coating of metal such as gold to the object but at best this requires a substantial complication of sample preparation procedures. Direct scanning of an object with an electron beam requires that the object be disposed in a vacuum chamber. This complicates the structure and slows operation as evacuation of the chamber, prior to each examination of a specimen, requires a significant amount of time. The need to maintain a vacuum at the surface of the object being scanned generally prevents use of this kind of system for analysis of living tissue.
Each of the problems and limitations discussed above can be avoided by utilizing a second known type of scanning x-ray spectrometer in which the primary radiation source is an x-ray tube. X-rays generated by the tube are blocked from the object which is being examined except for those x-rays which travel along a single narrow collimator passage directed towards the object. A motor driven mechanical system moves the x-ray tube including the collimator in a predetermined pattern to accomplish the scanning operation.
Mechanical scanning is inherently very slow in comparison with electronic scanning. Exposure times are longer. Factors such as vibrations and positional imprecisions in moving mechanical parts detract from the accuracy of the information produced by the system. Efforts to minimize such factors require an extremely costly construction. Thus resolution of the problems discussed above with respect to electron beam scanning has heretofore been accomplished only by accepting the other equally serious problems and limitations inherent in mechanical scanning.
The present invention is directed to overcoming one or more of the problems set forth above.